[Tests -> All](deps): Bump Discord.Net in /Source/SammBot.Tests

Bumps [Discord.Net](https://github.com/discord-net/Discord.Net) from 3.12.0 to 3.13.0.
- [Release notes](https://github.com/discord-net/Discord.Net/releases)
- [Changelog](https://github.com/discord-net/Discord.Net/blob/dev/CHANGELOG.md)
- [Commits](discord-net/Discord.Net@3.12.0...3.13.0)

---
updated-dependencies:
- dependency-name: Discord.Net
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>

Source/SammBot.Library/SammBot.Library.csproj

@@ -7,7 +7,7 @@
     </PropertyGroup>
 
     <ItemGroup>
-      <PackageReference Include="Discord.Net" Version="3.12.0" />
+      <PackageReference Include="Discord.Net" Version="3.13.0" />
       <PackageReference Include="Microsoft.EntityFrameworkCore" Version="8.0.0" />
       <PackageReference Include="SkiaSharp" Version="2.88.6" />
     </ItemGroup>